For use as a tuning circuit, various configurations using active elements and reactance elements have hitherto been proposed and put to practical use.
A conventional tuning circuit utilizing an LC resonance for example has a gain depending on an LC circuit, which gain may vary upon the regulation of a tuning frequency. Furthermore, typical tuning circuits extract predetermined frequency components from input signals, so that there may usually occur an attenuation of a signal amplitude with the gain resulting in 1 or less at the tuning frequency. For this reason, if amplification is required, a separate amplifier circuit must be connected thereto for amplifying the signal amplitude.